This invention relates generally to semiconductor devices and, more particularly, to passivation of semiconductor devices.
As is known in the art, it is common to encapsulate a semiconductor device in a inert material to isolate the semiconductor device from its immediate environment. The environment which may contain oxygen, water, etc. may modify the electrical characteristics of the device if not so isolated.
In particular, for microwave semiconductor devices, such as metal semiconductor field effect transistors (MESFETS) comprised of Group III-V materials such as gallium arsenide it is generally known to deposit a passivating material such as silicon nitride, silicon monoxide, silicon dioxide, silicon oxynitride, or polymide over the surface of the device and to thus isolate exposed surfaces of the gallium arsenide from an external environment.
Silicon nitride is amongst the most widely used material for passivation layers on gallium arsenide since it is extremely chemically stable and has excellent barrier properties. Moreover, the silicon nitride can also provide a dielectric for metal insulator metal (MIM) capacitors commonly employed in monolithic microwave integrated circuits, for example. The most common approach used to deposit silicon nitride for MESFET applications, whether the MESFET is a discrete device or incorporated as part of a monolithic microwave integrated circuit, is to use a plasma enhanced chemical vapor deposition technique in which a plasma of reactant gases which contain silicon and nitrogen is provided at a temperature in the range of approximately 150.degree. C. to 300.degree. C. The gases are reacted within this temperature range to deposit silicon nitride on the semiconductor substrate.
Although silicon nitride films, as well as many of the other passivation films have highly desirable characteristics as passivants they also have several drawbacks. Amongst these drawbacks are that silicon nitride films generally change threshold voltages of field effect transistors. That is, the voltage in which the channel of the transistor is "pinched off" may vary after silicon nitride deposition. It has been observed that surface damage from energetic ion bombardment provided during the plasma enhanced deposition degrades performance of the transistor.
A problem which is also observed in MESFETS after encapsulation by a passivation material including the aforementioned silicon nitride is a undesirable reduction in the reverse breakdown voltage between the gate and drain electrode as well as between the drain and source electrodes. This effect is observed with all types of encapsulation materials commonly used. The amount of reduction in reverse breakdown voltage varies widely from one wafer to a subsequent wafer resulting in large variations in final breakdown voltage characteristics. This variation makes specification of individual devices difficult to provide. Moreover, decreases in reverse breakdown voltage provides concomitant decreases in power capabilities. In addition to the aforementioned reduced reverse breakdown voltage, passivation materials, such as silicon nitride can occasionally change other MESFET characteristics. One common electrical characteristic which is changed by the presence of the passivation is leakage current between the gate and drain electrodes when operated under reverse bias conditions. This characteristic is often unstable and can change in magnitude with operation of the transistor. In particular, increases in leakage current often occur after several minutes of operation of the device under a high gate current condition.